1. Field of the Invention
The present invention relates to an image reading apparatus and a method for reading an image, and particularly to an image reading apparatus which reads an original such as a photographic film by using light transmitted through or reflected from the original, and a method which is applicable to the image reading apparatus.
2. Description of the Related Art
A conventional image reading apparatus is known which obtains image data representing an original by irradiating light emitted from a light source section onto the original such as a photographic film, making the light transmitted through or reflected from the original incident on a reading sensor formed by a CCD sensor or the like through a focusing lens or the like, and converting analog signals output from the reading sensor into digital data. In an image reading apparatus of this type, even when no image is set, the amount of light incident on the reading sensor through the focusing lens generally varies. For example, the amount of light incident on a peripheral portion of the light-receiving surface of the reading sensor is lower than that incident on a central portion of the light-receiving surface due to irregularity in the amount of light emitted from a light source section or aberration of the focusing lens or the like. Further, there is dispersion in the sensitivities of the respective photoelectric conversion cells of the reading sensor.
An image represented by signals, which are output from photoelectric conversion cells of a reading sensor at a time of image reading, has shading such as unevenness in density caused by the above described irregularity of amounts of light due to the light source, the lens, or the like, or caused by the above-described dispersion in sensitivities of the photoelectric conversion cells. Therefore, conventionally, in order to obtain signals which are equivalent to those signals which are generated by precisely reading an original, so-called shading correction (also referred to as a light correction) has been performed (for example, see Japanese Patent No. 2556486 and the like). In this shading correction, a gain (a correction value) for a signal output from each of the photoelectric conversion cells is obtained on the basis of the signal output from each photoelectric conversion cell of the reading sensor in accordance with light from the light source in a state in which no original is set, and the signal is corrected for each cell by using the obtained gain.
An image reading apparatus which reads a color original is generally structured to have a color separation device such as color separation filters or a dichroic mirror which is disposed at a light receiving side of a reading sensor and separates incident light into a plurality of color components (such as R, G and B). Light of one of the color components which has been separated by the color separation device is incident on the light-receiving surface of the reading sensor. When the entire surfaces of the color separation filters disposed at the light receiving side of the reading sensor are not precisely and evenly made, for example, due to production error, or when there is slight dispersion in the angles of incidence of light onto different portions of the dichroic mirror, the spectral characteristic of the color separation device vary along a direction perpendicular to the optical axis of the incident light.
Therefore, the spectral characteristic of the reading device, including the color separation device and the reading sensor, varies at respective portions of a reading area of the reading device (for example, the wavelength corresponding to the peak of the spectral characteristic varies), and this results in shading such as irregularity of color in an image represented by signals output from each of the photoelectric conversion cells of the reading sensor. The intensity and the like of the above shading differ depending on the relationships between the spectral sensitivity characteristic of the reading device and the spectral absorption characteristic of a coloring material (such as a coupler for a photographic film or an ink for a printed image) of an original. (Hereinafter, this latter characteristic is simply referred to as “spectral absorption characteristic of an original”.)
FIGS. 1A and 1B show (exemplary) spectral absorption characteristics wherein the vertical axis represents light transmittance (or reflectance). For example, when an original has the spectral absorption characteristic represented by the solid line of FIG. 1A, and a wavelength range of the spectral sensitivity characteristic of a reading device, which should have been the spectral sensitivity characteristic represented by the broken line of FIG. 1A, is shifted at the peripheral portions and the like of a reading area as represented by the alternate long and short dashed line of FIG. 1A due to dispersion of the spectral sensitivity characteristic of the reading device, shading occurs at the peripheral portions and the like of the reading area in accordance with the difference between the spectral absorption characteristic of the original within the wavelength range of the spectral sensitivity characteristic represented by the broken line and the spectral absorption characteristic of the original within the wavelength range of the spectral sensitivity characteristic represented by the alternate long and short dashed line. Thus, the intensity and the like of shading differ in accordance with the spectral absorption characteristic of an original (in this case, the difference between the spectral absorption characteristic of the original in the two wavelength ranges).